Method of forward in situ combustion utilizing air-water injection mixtures



March 2, 1965 D. R. PARRISH ETAL METHOD OF FORWARD IN SITU COMBUSTION UTILIZING AIR-WATER INJECTION MIXTURES 2 Sheets-Sheet 1 Filed April 30, 1962 INVENTORS: FORREST F. CRAIG, Jr.

BY DAVID R. PARRISH ATTORNEY March 1965 D. R. PARRISH ETAL ,47

METHOD OF FORWARD IN SITU COMBUSTION UTILIZING AIR-WATER INJECTION MIXTURES Filed April 30, 1962 2 Sheets-Sheet 2 l is/50S .LNOEH NOLLSHGWOO SNISSVd HELLVM ONV HIV d0 OLLVB o 8 s 5 3 r0 T \o O -0 o m .Pi a) Q.

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0 o o o o o g m w INVENTORS:

FORREST E CRAIG, Jr. DAVID R. PARRlSH ATTORNEY IQH/dOS OILVH HBLVM-BIV GEXJEPN! United States Patent "ice 3,171,479 METHOD 9? FORWARD IN SITU COMBUT1ON UTELIZING AIR-WATER INJECTION MIXTURES David R. Parrish and Forrest F. Craig, Jr., Tulsa, Okla, assignors to Pan American Petroleum Corporation,

Tulsa, @lsla a corporation of Deiaware Filed Apr. 30, 1962, Ser. No. 190,920 8 Claims. (Cl. 165-41) The present invention is concerned with a novel method for conducting underground combustion operations. More specifically, it is directed to a method for carrying out a forward combustion process under conditions requiring substantially less air than is necessary for conventional methods.

Briefly, the process of our invention is based on the discovery that the air requirements for underground combustion can be materially reduced by the injection of airxater mixtures at reservoir pressures sufficiently high to maintain the boiling point of water in said reservoir at a temperature above the ignition point of the fuel therein. This process is applicable to any reservoir that is suited to forward combustion and is particularly applicable to reservoirs from which the oil can be, or has been, removed by ordinary water flooding methods.

In all instances, where present methods of forward combustion are used in oil recovery, very large quantities of air are required in proportion to the oil actually recovered. This is true because the combustion zone in conventional forward burning processes does not move forward from a given location until the fuel supply at that point has been exhausted. In the process, some of the oil is pushed ahead toward the producing well or Wells. However, a very substantial amount of oil must be burned. Because the combustion zone in a conventional forward burning process does not move until a fuel supply in that location is no longer available, large quantities of air are necessary to move the front forward. Since air injection costs ordinarily constitute a major item of expense in operations of this kind, it will be apparent that any appreciable reduction in the amount of air necessary for the job increases very substantially the economic incentive for recovering valuable gaseous and liquid hydrocarbon products by means of underground combustion.

Accordingly, it is an object of our invention to provide a means by which forward combustion in a hydrocarbon reservoir can be conducted with the use of less air than would ordinarily be required. It is also an object of our invention to provide a novel process for recovering oil from reservoirs that have previously been flooded with water or similarly treated. It is another object of our invention to provide a method in which the water injected with the air and located over a substantial distance ahead of the combustion front is maintained at a level above the ignition temperature of the oil in the reservoir, thereby requiring less air for operation of the process than is normally needed.

In the drawings,

FIGURE 1 is a diagram illustrating existing conditions in the reservoir between the injection and producing wells when operating in accordance with our invention.

FIGURE 2 is a plot showing primarily the effect of pressure on the injected air-water ratio required in the process of our invention.

In carrying out the process of our invention, it should first be pointed out that this method is applicable to a reservoir that can be burned by currently known forward combustion procedures. Reservoirs of this type generally have a minimum permeability of about 10 millidarcys and the oil contained therein has a minimum viscosity of about 0.5 cps. at reservoir conditions. The face of the 3,371,479 Patented Mar. 2, 1965 formation traversed by the injection well or wells is first ignited in a known manner such as, for example, by means of a gas or a liquid-fueled burner, an electrical heater, or by the use of suitable pyrophoric materials, a thermite bomb, or the like. When the reservoir immediately surrounding the Well bore is thus brought to ignition temperature, air or an equivalent oxygen-containing gas is injected in an amount suflicient to establish a definite combustion zone or front. When the front is formed in this manner, temperatures of the order of 1000 F. to 2500 F. are generated.

In a typical case, the face of a formation containing hydrocarbons is heated to a temperature of from about 800 to about 1200 F., after which air is injected for a period of from 2 to 4 weeks at the rate of, say, 1 million s.c.f. per day. At the end of this time, the combustion front has moved in a substantially circular pattern away from the injection well for a distance of to feet. When combustion has proceeded to this extent, an airwater mixture is then introduced into the hot formation (which has been elevated to the desired pressure) with the water being present initially in amounts less than 1 barrel for each to 2500 s.c.f. of air injected. In this way water useful in our process is added to the system without any possibility of lowering the temperature of the formation to a level too low to support combustion.

Preferably, after the combustion front has been established, as mentioned above, the reservoir pressure is increased. However, these steps, if desired, may be carried out in the reverse order. In this connection, it should be noted that in the majority of instances Where combustion is employed, the reservoir pressure is low, e.g., below about 100 p.s.i. Accordingly, to take advantage of the teachings of our invention, the reservoir pressure should be increased to a minimum value of about 250 p.s.i. in most cases. The maximum pressure will, of course, vary butin any event-is below that which would tend to promote channeling of fluids through the formation or cause uncontrolled fracturing of the latter. Such pressure may 'be generated by the use of high fluid injection rates. For example, air injection rates of from about 10,000 to about 100,000 s.c.f.h. through the use of high-capacity compressors may be used. Alternatively, this condition can be obtained by placing the necessary amount of back pressure on the producing well or Wells in a known fashion. A combination of high-capacity compressors, for example equipment having a capacity of 50,000 s.c.f.h., and placement of back pressure on pro ducing wells may be necessary, or in some cases desirable, in order to maintain the steam in the reservoir at a level above the ignition temperature of the oil. Typical operating pressures for the kinds of reservoirs contemplated herein range from about 250 to about 3500 p.s.i.

Once a proper reservoir pressure has been created, a mixture of air and water is next injected into the formation. Generally speaking, the injected air-water ratio may vary from about 100 to about 3000 s.c.f./'bbl. On contact with the combustion front the injected water flashes into steam which, as a result of the reservoir pressurewhich is, for example, at about 350 p.s.i.is at a temperature of about 435 F. This means that the boiling point of any water present in the immediate area is above the ignition temperature of the oil. The minimum permissible pressure and corresponding temperature employed will vary some with the characteristics of the oil, the permeability of the reservoir rock, and the flow characteristics of the reservoir fluids. Typically, the pressure should exceed about 250 p.s.i. and the corresponding temperature should exceed about 400 P. This temperature may be lower for the lighter, more reactive crudes, but should be higher for the heavier, less reactive crudes.

Besides raising the boiling point of water above the ignition temperature of the oil, pressure has an additional effect. Low temperature steam is not suitable for the complete removal of the heavier crudes. For the heavier or higher boiling hydrocarbons the vapor pressure increases faster with increasing temperature than is true in the case of water. Thus, at higher temperatures, the heavier crudes are removed effectively while at lower temperature these materials are removed only to a minor extent, if at all.

The principle of this feature of our invention is further illustrated diagrammatically in FIGURE 1 of the drawings, in which a reservoir temperature profile is plotted against distance from an injection well when mixtures of air and water are injected into the formation in accordance with our invention. It will be appreciated that the distances shown cannot be given in exact number of feet because FIGURE 1 is intended to represent a dynamic system, and as the combustion process moves farther and farther away from the injection well, the lengths of the specific regions shown change. Thus, with respect to the heat bank generated, the reservoir can be divided into five zones, the first of which is a portion cooled by the injection of water. Zone No. 2 represents an area in which the injected water increases in temperature up to its boiling point at reservoir conditions. Zone No. 3 is a high-temperature zone which contains superheated steam and in which combustion occurs. Zone No. 4 is the steam plateau, a zone of relatively constant temperature containing saturated steam. As the front moves farther out into the reservoir, the temepratures in Zones No. 3 and 4 tend to equalize. The shape of Zone No. 4, as well as the other zones referred to herein, depends, of course, on the reservoir pressure. As shown in dotted lines, the shape of Zone 4 is typical of a low-pressure system, for example, one having a pressure of about 100 p.s.i., whereas the one shown in solid lines represents the same zo'ne under reservoir pressure conditions of about 1500 psi. In this regard, the dotted line in Zone N 0.2 helps to define the area involved in going from injection water temperature at low reservoir pressure to the boiling point of water at said pressure. Zone No. 5 is the unburned reservoir ahead of the heat front. At point (a), water flashes into steam. At point (0), steam condenses and the resultant condensate cools to reservoir temperature. The lengths of the arrows in FIGURE 1 indicate the relative velocities of the different heat fronts. Thus, Zone No. 4, containing saturated steam, grows by withdrawing or soaking up heat from Zone No. 3. For sysems at low pressure, high-temperature Zone No. 3 can be preserved only by injecting enough high-cost compressed air to generate suflicient heat to compensate for the heat flowing from Zone No. 3 to Zone No. 4.

The velocity of heat fronts (a) and (b) can be varied by changing the air-water ratio. Desirably, the ratio should be such that the velocity of front (a) is equal to that of front ([2). This is accomplished by injecting just enough air so that the heat liberated by the combustion provides the latent heat of vaporization for the injected water-crossing front (a). Since the latent heat of vaporization of water decreases with increasing pressure, less air is needed at high pressures. The effect is particularly noticeable at about 3206 p.s.i.a., the critical point of water, since the latent heat of water at that pressure is zero. If insufiicient air is injected, the high-temperature zone will shrink or disappear and the maximum temperature in the system is the boiling point of water at the pressure of the reservoir. In systems at low pressure, the temperature may thus decline below the ignition temperature and combustion stops. Such an operation will degenerate into an ineffective hot water flood and ultimately the heat bank disappears entirely. This phenomenon places a limit on the relative amounts of water and air that can be used. Since on a heat transporting basis water is cheaper and more efficient than compressed, air it is desirable to use as little air as possible. Accordingly, any procedure by which less air is required tends to improve the economics of this type of recovery method.

In the process of our invention we have discovered an unexpected advantage of pressure. At high pressures, the high-temperature zone, i.e., Zone No. 3 of FIGURE 1, may in time disappear, but combustion will continue in the steam bank which is composed of Zones No. 3 and No. 4, thereby permitting the process of our invention to be effectively maintained.

A further advantage of our invention resides in the fact that with systems which are above the critical pressure of water (3206 p.s.i.a.), steam has no latent heat of vaporization. The theft or withdrawal of heat from the high-temperature zone to the saturated steam zone is reduced and air requirements drop markedly. Since the aforesaid critical pressure corresponds to the critical temperature of water (705 F.), even the most unreactive crudes burn and even the heaviest crudes vaporize in the presence of such steam.

In measuring the value of our invention, it should be pointed out that few, if any, previously water-flooded reservoirs can be subjected to conventional forward combustion economically. Many of these reservoirs cannot support combustion because too little fuel is avail-able. In other cases, while sufl'icient oil is present to support combustion, little, if any, remains to be recovered. Since the process of our invention requires less air than conventional methods, the fuel requirements are smaller. Accordingly, our invention, both from the standpoint of economy and operability, is applicable where ordinary underground forward combustion methods cannot be used.

We have mentioned briefly above that air-water mix tures are used in our invention and that with increased pressures less air or oxygen-containing gas is required. The air-water ratio and the manner in which this ratio changes with changing reservoir pressures depends primarily on the latent heat of vaporization of water. In a typical steady-state system this relationship of air-water ratio to the reservoir pressure involved in carrying out our invention is expressed in Curve A, appearing in FIG- URE 2, in which the injected air-water ratio in terms of s.c.f. per barrel is plotted against reservoir pressure. This plot (Curve A), which assumes no heat loss or fillup of the pore space with water back of the combustion front, shows the material effect pressure has on the air requirements and that at and above the critical pressure of water the utilization of air in the process is extremely efilcient.

In practice, some loss of heat to surrounding rock strata occurs. The degree of heat loss depends primarily upon the thickness of the formation being burned and the velocity with which the combustion front moves through the formation. Heat losses decrease as the formation thickness and/or the combustion zone velocity increase. If heat loss is not to result in extinguishment of combustion, some additional air should be used to compensate for heat losses. Typically, about 25 percent more air must enter the combustion zone than is indicated by Curve A of FIGURE 2. In few, if any, cases, however, can airwater-atatios of less than 500 s.c.f./bbl. be employed.

Curve B shows the relationship of injected air-water ratios to pressure in a typical reservoir where heat losses are generally of the order of about 25 percent, as pointed out above. Accordingly, in determining the particular air-water ratios suitable for a given reservoir pressure, such ratios will generally lie within the band between Curves A and B. With a thick formation, for example, from about 75 to about feet, the air-water ratios will lie closer to Curve A, whereas in the case of a thin reservoir, e.g., from about 10 to about 15 feet, the ratios will be found to lie closer to Curve B.

We desire the air-water ratio entering the combustion zone to be between Curves A and B shown in FIGURE 2.,

However, it should be pointed out that a portion of the injected air and water is required to fill the reservoir pore space between the injection well bore and the advancing combustion zone. The ratio of injected air and water contributing to fill-up of this reservoir space is frequently not the same as the air-water ratio injected. The volume of the injected air and water consumed in this manner varies with the reservoir pressure and with the relative The process of our invention is illustrated by the results in the table below. These data were all obtained from tests on a rock having about percent porosity. Crude oil from the S1055 Field. Nebraska, having a gravity of about 38 AII was used. The rocks were first saturated with crude oil and simulated connate water and then water-flooded so that the oil saturation at the beginning of thermal recovery was about 40 percent pore permeability characteristics of the formation. In formavolume m all cases.

Table Injected Oil AOR Mr Run Average Atr-\\utcr Recovery, Required, No. Operation Pressure, ltattn. Ierccnt SCF r p.s.i.a. SCF/Ilbl. Oll-ln- Dbl. 0 OH place Produced (1)".-. Ilot (400 F.) Watcrflood 1,000 0 l0 (2). Forward Combustion..- 600 m 41.7 40,000 (:i).... Combustlon-wutcrlloutl- I00 250 Combustion ecnsed;n0 subsequent oil recovery 1,000 2,000 71. 13,500 2,000 I, 000 74. 2 It, (300 3,000 750 SI. 5 T, .500 3, 2l0 250 85. I 6, 820

tions having what is termed poor relative permeability characteristics, the ratio of air to water required to fill up the reservoir pore space back of the combustion zone is so low that the air-water ratio entering the combustion zone is higher than the ratio injected at the input well. Therefore, to assure that the air-water ratio entering the combustion zone lies between Curves A and D. the injected air-water ratio at the input well in formations with poor relative permeability characteristics should be below Curve A. Numerous calculations and theoretical considcrations have indicated that Curve C, shown on FIG- URE 2, represents the lower limit of injected air-water ratios such that, accounting for fill-up, the air-water ratio entering the combustion zone lies between Curves A and B. On the other hand, when the relative permeability characteristics of the formation under consideration are good. the ratio of air to water required for fill-up of the pore space back of the combustion zone can be near the injected ratio so that the injected ratio generally would be between Curves A and I3. Accordingly, for any given case the relative permeability characteristics of the formation and the extent of heat loss determine in large part the injected air-water ratio required at a given pressure. However, this injected ratio and reservoir pressure will generally define a point lying between Curves B and C.

While in general the air-water ratio to be employed at a given reservoir pressure can be determined to a substantial degree by reference to FIGURE 2, other factors. such as porosity, relative permeability characteristics of the reservoir rock. and the original temperature of the reservoir also must be taken into consideration.

Since the injected air and water will fill up and occupy the necessary volume as Zones No. l and 2 grow. the airwater ratio delivered at front (a), as previously discussed. can differ markedly from the air-water ratio injected at the input well. This difference is controlled by the aforementioned factors. In the majority of instances, however, the air-water ratio that can be employed will be found to vary from about 2500 s.c.f. per barrel to about 100 s.c.f. per barrel over a pressure range of from about 250 to about 3206 p.s.i. respectively.

An additional factor which should he considered is that the maximum pressure employed for any given case will depend on the depth of the formation to be burned and to that extent will determine or limit the variation in airwatcr ratios applicable for such case. Shallow formations, obviously, cannot be subjected to extreme fluid pressures because of the likelihood of causing undesired fractures. FIGURE 2 gives an indication of the air-water ratios suitable for a given reservoir pressure over contemplatcd operating conditions.

Three types of tests are illustrated in the table. In Run No. l, the reservoir was flooded with hot water having a temperature of about 400 F. Since the system pressure of 1000 p.s.i.a. corresponded to a higher boiling point temperature, no steam was formed in this case. Only about 10 percent of the oil-in-place at the beginning of the run was recovered. Run No. 2 was a conventional forward combustion operation. About 41.7 percent of the oil-in-place was recovered while using about 46,000 s.c.f. of air per barrel'of oil produced. Runs 3-7 were all combustion-waterllood experiments. Combustion was initiated and the combustion front was propagated a few feet through the reservoir rock using air alone. Simultaneous air-water injection was then started and was continued until the reservoir was completely burned through, or until oil production ceased. The primary variable in these runs were the mean reservoir pressure and the injected air-water ratio employed.

Run No. 3 was conducted using an injected air-water ratio lower than is indicated by the curves of FIGURE 2. Shortly after water injection was commenced, combustion ceased as evidenced by the increase in oxygen content of the produced gas from near zero to essentially the.composition of air. Oil production dwindled rapidly as combustion ceased and essentially none of the oil in the unburned part of the reservoir was recovered even though injections were continued for a long period.

The remaining runs, Nos. 4-7, were conducted using air-water ratios within the prescribed region of FIG- URE 2. In all cases combustion continued throughout the reservoir. It is apparent that oil recovery was substantially better than was obtained by waterfiooding or combustion alone. It is also apparent that considerably less air was required than was needed for conventional forward combustion. A further advantage of high pressure is illustrated by the increase in oil recovery and the decrease in air requirements as pressure increases, hence lower air-water ratios may be employed as illustrated in FIGURE 2.

We claim:

1. In a process for the recovery of hydrocarbons from an underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and a producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well,

subjecting said reservoir to a fluid pressure of from about 250 p.s.i. to a pressure just below that required to fracture said reservoir,

next propagating said combustion zone toward said producing well by injecting a mixture of oxygencontaining gas and water in which a combination of the ratio of oxygen to water at the prevailing reservoir pressure defines a point falling substantially within the shaded area between Curves B and C of FIGURE 2, sustaining said combustion zone by varying the oxygen requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons, and

thereafter recovering hydrocarbons from said producing well. 2. In a process for the recovery of hydrocarbons from a previously waterflooded underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and a producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well,

subjecting said reservoir to a fluid pressure of from about 250 p.s.i. to a pressure just below that required to fracture said reservoir,

next propagating said combustion zone toward said producing well by injecting a mixture of oxygencontaining gas and water in which a combination of the ratio of oxygen to water at the prevailing reservoir pressure defines a point falling substantially within the shaded area between Curves B and C of FIGURE 2, sustaining said combustion zone by varying the oxygen requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons,

continuing the injection of said mixture to propagate said zone toward said producing well, and thereafter recovering hydrocarbons from said producing well. 3. In a process for the recovery of hydrocarbons from an underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and a producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well,

subjecting said reservoir to a fluid pressure of from about 250 p.s.i. to a pressure just below that required to fracture said reservoir,

next propagating said combustion zone toward said producing well by injecting a mixture consisting essentially of air and water in which a combination of the ratio of air to water at the prevailing reservoir pressure defines a point falling substantially within the shaded area between Curves I3 and C of FIG- URE 2, sustaining said combustion zone by varying the air requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons,

continuing the injection of said mixture to propagate said zone toward said producing well, and thereafter recovering hydrocarbons from said produeing well. 4. In a process for the recovery of hydrocarbons from an underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and a producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well,

subjecting said reservoir to a fluid pressure of from about 250 p.s.i. to a pressure just below that required to fracture said reservoir,

next injecting a mixture of an oxygen-containing gas and water in which the oxygen and the water are present in a ratio of from about 20 s.c.f. per barrel to about 550 s.c.f. per barrel of water, sustaining said combustion zone by varying the oxygen requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons,

continuing the injection of said mixture to propagate said zone toward said producing well, and thereafter recovering hydrocarbons from said producing well.

5. In a process for the recovery of hydrocarbons from an underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and a producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well, subjecting said reservoir to a fluid pressure of from about 250 p.s.i. to a pressure just below that required to fracture said reservoir, next propagating said combustion zone toward said producing well by injecting a mixture of air and water in a ratio of from about s.c.f. per barrel to about 2700 s.c.f. per barrel of water, sustaining said combustion zone by varying the air requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons, continuing the injection of said mixture to propagate said zone toward said producing well, and thereafter recovering hydrocarbons from said producing well. 6. The process of claim 5 in which the combustion is effected in a previously waterflooded reservoir.

7. In a process for the recovery of hydrocarbons from an underground reservoir by means of combustion, said reservoir being penetrated at spaced points by an injection well and at producing well, the improvement which comprises,

initiating a zone of combustion in said reservoir at a point adjacent the face of said injection well, said reservoir being at sufficient depth so that pressures may be employed ranging from the critical pressure of water to a pressure just below that required to fracture said reservoir, next propagating said combustion zone toward said producing well by injecting a mixture of an oxygencontaining gas and water in which the oxygen-water ratio ranges from about 20 s.c.f. per barrel to about 550 s.c.f. per barrel of water, sustaining said combustion zone by varying the oxygen requirements therefor inversely with said pressure, whereby the steam thus generated in said reservoir is above the ignition temperature of said hydrocarbons,

continuing injection of said mixture to propagate said zone toward said producing well, and

thereafter recovering hydrocarbons from said producing well.

8. The process of claim 7 in which the combustion is effected in a previously waterflooded reservoir.

References Cited in the file of this patent UNITED STATES PATENTS 2,642,943 Smith et al. June 23, 1953 2,788,071 Pelzer Apr. 9, 1957 3,024,841 Willman Mar. 13, 1962 

1. IN A PROCESS FOR THE RECOVERY OF HYDROCARBON FROM AN UNDERGROUND RESERVOIR BY MEANS OF COMBUSTION, SAID RESERVOIR BEING PENETRATED AT SPACED POINTS BY AN INJECTION WELL AND A PRODUCING WELL, THE IMPROVEMENT WHICH COMPRISES, INITIATING A ZONE OF COMBUSTION IN SAID RESERVOIR AT A POINT ADJACENT THE FACE OF SAID INJECTION WELL, SUBJECTING SAID RESERVOIR TO A FLUID PRESSURE OF FROM ABOUT 250 P.S.I. TO A PRESSURE JUST BELOW THAT REQUIRED TO FRACTURE SAID RESERVOIR, NEXT PROPAGATING SAID COMBUSTION ZONE TOWARD SAID PRODUCING WELL BY INJECTING A MIXTURE OF OXYGENCONTAINING GAS AND WATER IN WHICH A COMBINATION OF THE RATIO OF OXYGEN TO WATER AT THE PREVAILING RESERVOIR PRESSURE DEFINES A POINT FALLING SUBSTANTIALLY WITHIN THE SHADED AREA BETWEEN CURVES B AND C OF FIGURE 2, SUSTAINING SAID COMBUSTION ZONE BY VARYING THE OXYGEN REQUIREMENTS THEREFOR INVERSELY WITH SAID PRESSURE, WHEREBY THE STEAM THUS GENERATED IN SAID RESERVOIR IS ABOVE THE IGNITION TEMPERATURE OF SAID HYDROCARBON, AND THEREAFTER RECOVERING HYDROCARBON FROM SAID PRODUCING WELL. 